The Features And Mechanism Of Ammonium Removal By Zeolite In The Presence Of Humic Acid

Zeolite has high cation exchange capacity and selective sorption for NH4+. Theresearch about the removal features of NH4+by natural zeolite has been paid muchattention recently. But the features and mechanism of NH4+removal by zeolite in thepresence of humic acid has not been reported in detail. In this study, the naturalzeolite and modified zeolite were used as the sorbents to study the sorption process ofNH4+in the presence of humic acid. The scanning electron microscopy (SEM), BETspecific surface area and pore volume analysis were used to characterize the zeolite.Several kinetic models and isotherm models were used to understand the sorptionmechanism. The main conclusions are as follows:(1) When natural zeolite was used as sorbent, the pseudo-second-order kineticmodel and the Langmuir–Freundlich isotherm fited the experimental data well.Existence of humic acid (40mg/L) significantly decreased the initial sorption rate ofNH4+(4.99to2.73mg/(g·h)). But insignificant effect on NH4+maximum sorptioncapacity was found in the experiments.(2) With the increase of humic acid concentration (0to40mg/L), the negativeeffect of humic acid on the removal of NH4+by zeolite was more obvious when theinitial concentration of NH4+was5mg/L, the removal efficiency of NH4+decreasedfrom93.0%to85.1%. When the initial NH4+concentration was50mg/L, the removalefficiency of NH4+decreased from49.1%to45.5%, the negative effect of humic aciddecreased. The reason was that the driving force from concentration gradientincreased, this could be beneficial to overcome the resistances of NH4+transferbetween the aqueous and the solid phases, which resulted in the decrease of thenegative effect of humic acid.(3) High aqueous Ca2+concentration (100mg/L) can decrease the negative effect of humic acid on NH4+sorption. Exisitence of Ca2+reduces the negative charge ofhumic acid and weak the ion exchange process between the humic acid andammonium. In addition, Ca2+and humic acid can form the precipitate of calciumhumate, which reduces the concention of Ca2+and humic acid in the solution.（4）Zeolite modified by CaCl2,FeCl3and KAl(SO4)2can increase the removalefficiency of humic aicd. The zeolite modified by KAl(SO4)2has the highest removalefficiency of humic acid. With the increase of KAl(SO4)2concentration (0.005mol/Lto0.1mol/L), the removal efficiency of NH4+decreased from91.0%to82.3%, but theremoval efficiency of humic acid increased from72.8%to94.6%. The zeolitemodified by0.05mol/L KAl(SO4)2has the highest potential to remove NH4+andhumic acid simultaneously.